Personal fitness and health monitoring devices, referred to as biometric monitoring devices herein, may include a variety of different sensors that are used to provide feedback regarding various physiological characteristics of a person. Such sensors may include, but are not limited to, accelerometers used to track activity intensity and steps taken, barometric pressure sensors used to track elevation gain, galvanic skin response sensors for detecting perspiration, and heart rate sensors that track heart rate and sometimes other physiological characteristics.
One type of sensor that may be used in biometric monitoring devices is an optical physiological parameter measurement device or sensor, such as a photoplethysmographic (PPG) heart rate sensor, that uses a photo-emitter and a photodetector. In a PPG sensor, a photo-emitter located adjacent to a person's skin emits light that is diffused within the person's skin and then reflected back out of the person's skin and into a photodetector. The amount of the emitted light that is diffused/reflected back out of the person's skin and into the photodetector will vary with the person's pulse rate. This is because the person's blood vessels will expand and contract in response to every heartbeat, e.g., as the heart pushes blood into the blood vessels, they expand—when the heart is no longer exerting pressure on the blood, the blood vessels contract. As the blood vessels expand and contract, the volume occupied by the blood vessels fluctuates, which affects the amount of light that is reflected back into the photodetector. This variation is cyclic and, by analyzing the amount of reflected light that is detected by the photo-emitter, it is possible to determine the frequency of that cyclic behavior and, therefore, the person's heart rate.
The concepts discussed herein may be applied to PPG sensors, as well as to other optical physiological parameter measurement devices, such as optically-based physiological sensors that utilize a light source coupled with a photodetector configured to detect light from the light source that is reflected off of a person's body or reflectively diffused from the person's body (such sensors may be classified as “reflective” sensors since they operate using light that is emitted and then reflected from a person's body); this disclosure is to be understood as not being limited to only PPG sensors or only PPG heart rate sensors. It is also to be understood that a PPG heart rate sensor is only one form of PPG sensor, and that other PPG sensors may measure other physiological parameters in place of, or in addition to, heart rate.